Physiological measurement systems employed in healthcare often feature visual and audible alarm mechanisms that alert a caregiver when a patient's vital signs are outside of predetermined limits. For example, a pulse oximeter, which measures the oxygen saturation level of arterial blood, indicates oxygen supply. A typical pulse oximetry system has a sensor that provides a signal output to a pulse oximeter monitor. The sensor has an emitter configured with both red and infrared LEDs that project light through a fleshy medium to a detector so as to determine the ratio of oxygenated and deoxygenated hemoglobin light absorption. The monitor has a signal processor, a display and an alarm. The signal processor inputs the conditioned and digitized sensor signal and calculates oxygen saturation (SpO2) along with pulse rate (PR), as is well-known in the art. The display provides a numerical readout of a patient's oxygen saturation and pulse rate. The alarm provides an audible indication when oxygen saturation or pulse rate are outside of predetermined limits.
Another pulse oximetry parameter is perfusion index (PI). PI is a measure of perfusion at the pulse oximetry sensor site comparing the pulsatile (AC) signal to the non-pulsatile (DC) signal, expressed as a percentage ratio. An example is the PI Delta Alarm™ feature of the Radical 7™ Pulse CO-Oximeter™ available from Masimo Corporation, Irvine, Calif., which alerts clinicians to specified changes in PI. In particular, PI Delta indicates if PI at a monitored site decreases by a specific level (delta) over a specified window of time, with both variables selectable by the user within predetermined ranges.
Tracking a series of desaturations over time is one metric that is derived from SpO2 that is well-known in the art. See, e.g., Farney, Robert J., Jensen, Robert L.; Ear Oximetry to Detect Apnea and Differentiate Rapid Eye Movement (REM) and Non-REM (NREM) Sleep: Screening for the Sleep Apnea Syndrome; Chest; April 1986; pages 533-539, incorporated by reference herein. Traditional high and low SpO2 alarm limits alert clinicians to saturation levels that exceed user-selected thresholds, and these thresholds are typically established at a considerable change from the patients' baseline saturation level. However, in select patient populations, substantial desaturation events that exceed a typical low alarm limit threshold may be preceded by a cycle of transient desaturations over a limited timeframe. The ability to alert clinicians to a cycle of these smaller desaturations provides an earlier indication of a potential significant decline in the patient's status and the need for more focused monitoring and/or a change in treatment. An example is the Desat Index Alarm™ feature of the Radical 7™, mentioned above, which enables clinicians to detect an increasing quantity of smaller desaturations that may precede declining respiratory status. Desat Index is a measure responsive to patients that experience a specific number of desaturations beyond a defined level from the patient's baseline saturation over a specific window of time, with each of these variables selectable by the user within predetermined ranges.
A physiological parameter that can be measured in addition to, or in lieu of, SpO2 is respiration rate (RR). A respiration rate monitor utilizes a body sound sensor with piezoelectric membranes particularly suited for the capture of acoustic waves and the conversion thereof into electric signals. To detect body sound, the piezoelectric membranes are used as mechano-electric transducers that are temporarily polarized when subject to a physical force, such as when subjected to the mechanical stress caused by the acoustic waves coming from the inside of a patient's body. The body sound sensor is typically attached to the suprasternal notch or at the lateral neck near the pharynx so as to detect tracheal sounds. A sound sensor is described in U.S. Pat. No. 6,661,161 entitled Piezoelectric Biological Sound Monitor With Printed Circuit Board, incorporated by reference herein. A respiration rate monitor is described in U.S. patent application Ser. No. 11/547,570 entitled Non-Invasive Monitoring of Respiratory Rate, Heart Rate and Apnea, incorporated by reference herein.